1. Field of the Invention
The present embodiment relates to a driving method of a solid-state imaging device.
2. Description of the Related Art
A solid-state imaging device is generally constituted by pixels arranged like a two-dimensional array, a vertical scanning circuit and a horizontal scanning circuit respectively driving photoelectric conversion signals in the pixels, a reading circuit, and the like. Further, each pixel is generally constituted by a photoelectric conversion unit, a transfer transistor for transferring a signal charge, a reset transistor for resetting the photoelectric conversion unit, and the like. A charge accumulation operation is started at the point of changing the transfer transistor from a conduction state to a non-conduction state by resetting the photoelectric conversion unit with use of the reset transistor, and is terminated at the point of starting the charge transfer from the photoelectric conversion unit to an FD (floating diffusion) region with use of the transfer transistor. Such a period is called an accumulating period. If the photoelectric conversion unit cannot be reset completely and thus a residual charge still exists, a residual image and a noise are caused.
Japanese Patent Application Laid-Open No. 2000-201300 discloses a technique of, in case of performing a reset operation, putting both a reset transistor and a transfer transistor into conduction. More specifically, when the reset operation is started, the transfer transistor is first put into conduction, and the reset transistor is then put into conduction. By such an operation, residual charges in a photoelectric conversion unit are reduced, and thus residual images and noises are reduced.
To cope with an increase of the number of pixels and an increase of operation speed in the field of digital cameras, each pixel in the solid-state imaging device has been decreased in size year by year. In association with such a tendency, a power supply voltage has been decreased, whereby it becomes necessary to design the solid-state imaging device in a further microfabrication size. Incidentally, since the elements to be provided in the pixel are arranged in mutually close to others in accordance with miniaturization of the pixel size, a parasitic capacitance is increased. For example, when the photoelectric conversion unit is reset by the method disclosed in Japanese Patent Application Laid-Open No. 2000-201300, a change of the gate voltage in case of putting the reset transistor into a non-conduction state changes the voltage in the FD region through the above parasitic capacitance, whereby a problem that the residual charges are increased becomes serious. This is because a change of the potential in the FD region acts on a direction in which it is difficult for the transfer transistor to transfer the charge.
Further, in such a state that it is difficult to transfer the charge, dullness of a pulse to be applied to the gate of the transfer transistor actually affects a quantity of the residual charges due to a time constant of the gate line of the transfer transistor extending in the row direction. Since the dullness of the pulse to be applied to the gate line is different according to a position on the transfer line, a noise output may be slant along the horizontal direction of a screen. Here, a noise output waveform having gradation in a direction parallel to the horizontal scanning direction of the solid-state imaging device is called “horizontal shading”. Since the horizontal shading may appear in un-ignorable way particularly in a dark portion of a taken image and a portion that brightness of a subject is low, it is desired to reduce the horizontal shading like this.
Therefore, an object of the embodiment of the present invention is to provide a driving method of a solid-state imaging device, by which it is possible to reduce a horizontal shading which is caused by incompleteness of resetting.